Transcriptional Analysis of the Multicopy hao Gene Coding for Hydroxylamine Oxidoreductase in Nitrosomonas sp. Strain ENI-11

Abstract

The nitrifying bacterium Nitrosomonas sp. strain ENI-11 has three copies of the gene encoding hydroxylamine oxidoreductase (hao₁, hao₂, and hao₃) on its genome. Broad-host-range reporter plasmids containing transcriptional fusion genes between hao copies and lacZ were constructed to analyze the expression of each hydroxylamine oxidoreductase gene (hao) copy individually and quantitatively. β-Galactosidase assays of ENI-11 harboring reporter plasmids revealed that all hao copies were transcribed in the wild-type strain. Promoter analysis of hao copies revealed that transcription of hao₃ was highest among the hao copies. Expression levels of hao₁ and hao₂ were 40% and 62% of that of hao₃ respectively. Transcription of hao₁ was negatively regulated, whereas a portion of hao₃ transcription was read through transcription from the rpsT promoter. When energy-depleted cells were incubated in the growth medium, only hao₃ expression increased. This result suggests that it is hao₃ that is responsible for recovery from energy-depleted conditions in Nitrosomonas sp. strain ENI-11.